In accordance with EPA regulations, an above-ground storage tank system for containing fuels, potentially hazardous fluids, or any other liquids that would create environmental problems if released into the ground must include a secondary containment device capable of storing at least 110% of the fluid that can be contained in the storage tank. The secondary containment device is typically comprised of a dike that at least partially surrounds the tank to prevent ground contamination that may occur from leaks and overfills.
Several containment system designs are known for storing fluid that was spilled or overfilled from an internal storage tank. U.S. Pat. No. 5,203,386 ("the '386 patent") discloses a storage system in which a secondary container having an attached hood completely surrounds an internal storage tank. The hood over the dike prevents rain or ambient precipitation and trash from entering the dike. As shown in FIGS. 1 and 2 herein, which correspond with FIGS. 1 and 2 of the '386 patent, the internal storage tank is filled by opening a door 10 on housing 12 formed at the top of the structure to gain access to one of the internal tank ports 20, 22, 24. The housing 12 is located above input ports but is not connected to the internal tank. If fluid leaks from one of the ports or is spilled within the housing 12, it runs along the exterior of the internal tank 14, within the sloping side walls 16 of hood 18, and into the dike 26.
U.S. Pat. No. 4,895,272 ("the '272 patent") describes another liquid storage system having a roof structure that extends from a point along the upper side of the internal storage tank to the sides of the dike. As shown in FIGS. 3-4, drainage structure 40 is a passageway that extends from the side of housing 38, through the roof structure 34, and into storage space 42 in the external containment vessel 32. If liquid is spilled or is overflowing from the internal tank, it runs within the passageway 46 and along the exterior of the internal storage tank.
In many applications that require an above-ground storage tank apparatus, it would be advantageous to recover any spilled or overflowing fluid for use. However, this is not possible with the liquid storage system described in the '272 patent, because the roof structure does not provide a liquid-tight seal to prevent spilled liquid from contamination. In most circumstances, spilled liquid cannot be re-used if it contacts with water in the external containment vessel 32, and it then must be disposed as a hazardous material. As for the containment system described in the '386 patent, it is difficult for an operator to detect whether any fluid has leaked from an input port, because there is no bottom portion to the housing 28 to collect the spilled fluid.
Even if the spilled liquid does not become contaminated in the systems disclosed in the '272 and '386 patents, it must be drained or pumped from the dikes. The interior of the dike must then be cleaned, which can be a difficult and time-consuming process. On most occasions, only a small quantity of fluid leaks from an input port of the internal storage tank. Accordingly, there is a need for an above-ground storage tank apparatus that overcomes the problems of contamination and the labor-intensive process required for recovering fluid when only a small portion of fluid leaks from an input port of the internal tank.
For some applications, it is also advantageous to pressure test the storage space between external containment vessel and the internal storage tank. Pressure testing assures that the external containment area is properly sealed such that any spilled fluid does not become contaminated. However, the containment systems described in both the '272 and the '386 patents do not readily allow an operator to conduct pressure testing. Accordingly, there is also a need for a storage tank apparatus that provides for pressure testing.